Low frequency pulse tube system with oil-free drive

ABSTRACT

A pulse tube system for generating refrigeration for uses such as in magnetic resonance imaging systems wherein an oil-free compressor operating at a higher frequency generates pulsing gas which undergoes a frequency reduction and drives the pulse tube system at a more efficient lower frequency.

TECHNICAL FIELD

This invention relates generally to low temperature or cryogenicrefrigeration and, more particularly, to pulse tube refrigeration.

BACKGROUND

A recent significant advancement in the field of generating lowtemperature refrigeration is the pulse tube system or cryocooler whereinpulse energy is converted to refrigeration using an oscillating gas.Such systems can generate refrigeration to very low levels sufficient,for example, to liquefy helium. One important application of therefrigeration generated by such cryocooler system is in magneticresonance imaging systems.

One problem with conventional cryocooler systems is contamination of thepulsing gas by the pulse generating equipment. Moreover, a source ofinefficiency is a mismatch between the most efficient operatingfrequency of the cryocooler system and the most efficient operatingfrequency of the pulse generating system.

Accordingly it is an object of this invention to provide an improvedcryocooler or pulse tube system which has reduced contaminationpotential and more efficient operation.

SUMMARY OF THE INVENTION

The above and other objects, which will become apparent to those skilledin the art upon a reading of this disclosure, are attained by thepresent invention, one aspect of which is:

A method for operating a low frequency cryocooler system comprising:

(A) generating pulsing gas at a frequency of at least 25 hertz bycompressing a gas using a moving element moving proximate a surroundingwall wherein no oil is employed between the moving element and thesurrounding wall;

(B) passing the pulsing gas through a frequency modulation valve andreducing the frequency of the pulsing gas to produce lower frequencypulsing gas; and

(C) passing the lower frequency pulsing gas to a regenerator which is inflow communication with a thermal buffer tube.

Another aspect of the invention is:

A low frequency cryocooler system comprising:

(A) a compressor having a discharge and having a moving elementproximate a surrounding wall wherein no oil is employed between themoving element and the surrounding wall;

(B) a regenerator, a frequency modulation valve, discharge conduitextending from the discharge to the frequency modulation valve, andregenerator input/output conduit extending from the frequency modulationvalve to the regenerator; and

(C) a thermal buffer tube in flow communication with the regenerator.

As used herein the term “regenerator” means a thermal device in the formof porous distributed mass or media, such as spheres, stacked screens,perforated metal sheets and the like, with good thermal capacity to coolincoming warm gas and warm returning cold gas via direct heat transferwith the porous distributed mass.

As used herein the term “thermal buffer tube” means a cryocoolercomponent separate from the regenerator and proximate the cold heatexchanger and spanning a temperature range from the coldest to thewarmer heat rejection temperature for that stage.

As used herein the term “indirect heat exchange” means the bringing offluids into heat exchange relation without any physical contact orintermixing of the fluids with each other.

As used herein the term “direct heat exchange” means the transfer ofrefrigeration through contact of cooling and heating entities.

As used herein the term “frequency modulation valve” means a valve orsystem of valves generating oscillating pressure and mass flow at adesired frequency.

As used herein the term “discharge frequency modulating volume” meansthe total volume of the discharge conduit, and the reservoir ifemployed, extending from the compressor discharge to the frequencymodulation valve. The discharge frequency modulating volume may be from0.1 to 10 times the displacement volume of the compressor.

As used herein the term “suction frequency modulating volume” means thetotal volume of the suction conduit, and the reservoir if employed,extending from the frequency modulation valve to the compressor suction.The suction frequency modulation volume may be from 0.1 to 10 times thedisplacement volume of the compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of one preferred embodiment of theinvention wherein the compressor is a linear compressor and thefrequency modulation valve is a rotary valve.

FIG. 2 is a schematic representation of another preferred embodiment ofthe invention wherein the compressor is a linear compressor and thefrequency modulation valve is a control valve system.

The numerals in the Drawings are the same for the common elements.

DETAILED DESCRIPTION

The invention will be described in detail with reference to theDrawings. Referring now to FIG. 1, an oil-free compressor generates apulsing gas to drive the cryocooler or pulse tube system which comprisesregenerator 20 and thermal buffer tube 40. Oil-free compressors operateefficiently at high frequencies, typically at from 50 to 60 hertz. Inthe embodiment of the invention illustrated in FIG. 1 the oil-freecompressor is a linear compressor 1 driven by an electrically drivenlinear motor, i.e. axially reciprocating electromagnetic transducer 2.Another example of an oil-free compressor which may be used in thepractice of this invention is an oil-free guided rotary compressordriven by a rotary motor.

The oil-free compressor has a moving element proximate a surroundingwall. In the embodiment of the invention illustrated in FIG. 1 themoving element is piston 3 which is driven back and forth by linearmotor 2. Piston 3 reciprocates within the volume defined by casing orsurrounding wall 8 and is proximate surrounding wall 8 separatedtherefrom by clearance 7. There is no oil in clearance 7 between piston3 and surrounding wall 8. Instead, the linear compressor employs gasbearings or flexure suspensions to ensure facile motion of piston 3.

The reciprocating piston 3 generates gas having a pulsing or oscillatingmotion at the frequency of the alternating current power supplied of atleast 25 hertz and typically about 50 to 60 hertz. Check valve system 4,usually termed reed valves, converts the oscillating pressure wave toobtain a compression output at compressor discharge 5 which has smallfluctuations at its operating frequency. Examples of gas which may beused as the pulsing gas generated by the oil-free compressor in thepractice of this invention include helium, neon, hydrogen, nitrogen,argon, oxygen, and mixtures thereof, with helium being preferred.

The pulsing gas is cooled of the heat of compression in cooler 12 andpassed in discharge conduit 18 to frequency modulation valve 17 which,in the embodiment illustrated in FIG. 1, is a rotary valve. Rotary valve17 is driven by a motorized system which is not shown in FIG. 1.Preferably, as shown in FIG. 1, the high frequency pulsing gas indischarge conduit 18 passes through reservoir 13. The dischargefrequency modulating volume of discharge conduit 18 and reservoir 13serves to decouple the pulse rate between the compressor and thecrycooler by providing a steady gas supply at a relatively stablepressure to the valve. As the rotating part (not shown) of rotary valve17 rotates, the bores alternatively connect the compressor dischargeconduit 18 to the regenerator inlet/outlet conduit 62, and theregenerator inlet/outlet conduit 62 to the compressor suction conduit19. These alternating connections generate oscillating pressure and massflow thus a pressure-volume work at the rotation frequency of the valve17.

As the pulsing gas passes through the frequency modulation valve itsfrequency is reduced to the most efficient operating frequency of thecryocooler. The resulting lower frequency pulsing gas generally has afrequency less than 40 hertz, typically has a frequency less than 30hertz, preferably less than 10 hertz, most preferably less than 5 hertz.The lower frequency pulsing gas is then passed to regenerator 20 of thecryocooler or pulse tube system. Regenerator 20 is in flow communicationwith thermal buffer tube 40 of the pulse tube system.

The lower frequency pulsing gas applies a pulse to the hot end ofregenerator 20 thereby generating an oscillating working gas andinitiating the first part of the pulse tube sequence. The pulse servesto compress the working gas producing hot compressed working gas at thehot end of the regenerator 20. The hot working gas is cooled, preferablyby indirect heat exchange with heat transfer fluid 22 in heat exchanger21, to produce warmed heat transfer fluid in stream 23 and to cool thecompressed working gas of the heat of compression. Examples of fluidsuseful as the heat transfer fluid 22, 23 in the practice of thisinvention include water, air, ethylene glycol and the like. Heatexchanger 21 is the heat sink for the heat pumped from the refrigerationload against the temperature gradient by the regenerator 20 as a resultof the pressure-volume work generated by the compressor and thefrequency modulation valve.

Regenerator 20 contains regenerator or heat transfer media. Examples ofsuitable heat transfer media in the practice of this invention includesteel balls, wire mesh, high density honeycomb structures, expandedmetals, lead balls, copper and its alloys, complexes of rare earthelement(s) and transition metals. The pulsing or oscillating working gasis cooled in regenerator 20 by direct heat exchange with coldregenerator media to produce cold pulse tube working gas.

Thermal buffer tube 40 and regenerator 20 are in flow communication. Theflow communication includes cold heat exchanger 30. The cold working gaspasses in line 60 to cold heat exchanger 30 and in line 61 from coldheat exchanger 30 to the cold end of thermal buffer tube 40. Within coldheat exchanger 30 the cold working gas is warmed by indirect heatexchange with a refrigeration load thereby providing refrigeration tothe refrigeration load. This heat exchange with the refrigeration loadis not illustrated. One example of a refrigeration load is for use in amagnetic resonance imaging system. Another example of a refrigerationload is for use in high temperature superconductivity.

The working gas is passed from the regenerator 20 to thermal buffer tube40 at the cold end. Preferably, as illustrated in FIG. 1 thermal buffertube 40 has a flow straightener 41 at its cold end and a flowstraightener 42 at its hot end. As the working gas passes into pulsethermal buffer 40 it compresses gas in the thermal buffer tube andforces some of the gas through heat exchanger 43 and orifice 50 in line51 into the reservoir 52. Flow stops when pressures in both the thermalbuffer tube and the reservoir are equalized.

Cooling fluid 44 is passed to heat exchanger 43 wherein it is warmed orvaporized by indirect heat exchange with the working gas, thus servingas a heat sink to cool the compressed working gas. Resulting warmed orvaporized cooling fluid is withdrawn from heat exchanger 43 in stream45. Preferably cooling fluid 44 is water, air, ethylene glycol or thelike.

In the low pressure point of the pulsing sequence, the working gaswithin the thermal buffer tube expands and thus cools, and the flow isreversed from the now relatively higher pressure reservoir 52 into thethermal buffer tube 40. The cold working gas is pushed into the coldheat exchanger 30 and back towards the warm end of the regenerator whileproviding refrigeration at heat exchanger 30 and cooling the regeneratorheat transfer media for the next pulsing sequence. Orifice 50 andreservoir 52 are employed to maintain the pressure and flow waves inphase so that the thermal buffer tube generates net refrigeration duringthe compression and the expansion cycles in the cold end of thermalbuffer tube 40. Other means for maintaining the pressure and flow wavesin phase which may be used in the practice of this invention includeinertance tube and orifice, expander, linear alternator, bellowsarrangements, and a work recovery line connected back to the compressorwith a mass flux suppressor. In the expansion sequence, the working gasexpands to produce working gas at the cold end of the thermal buffertube 40. The expanded gas reverses its direction such that it flows fromthe thermal buffer tube toward regenerator 20. The relatively higherpressure gas in the reservoir flows through valve 50 to the warm end ofthe thermal buffer tube 40. In summary, thermal buffer tube 40 rejectsthe remainder of pressure-volume work generated by the compression andfrequency modulation system (which comprises the oil-free compressor andthe frequency modulation valve) as heat into warm heat exchanger 43.

The expanded working gas emerging from heat exchanger 30 is passed inline 60 to regenerator 20 wherein it directly contacts the heat transfermedia within the regenerator to produce the aforesaid cold heat transfermedia, thereby completing the second part of the pulse tube refrigerantsequence and putting the regenerator into condition for the first partof a subsequent pulse tube refrigeration sequence. Pulsing gas fromregenerator 20 passes back to rotary valve 17 and in suction conduit 19to suction 6 of compressor 1. Preferably reservoir 16 is employed onsuction conduit 19 and the suction frequency modulating volume ofsuction conduit 19 and reservoir 16 serves a purpose similar to that ofthe discharge frequency modulating volume.

FIG. 2 illustrates another embodiment of the invention. The elementscommon to the embodiments illustrated in FIGS. 1 and 2 will not bedescribed again in detail. In the embodiment illustrated in FIG. 2 therotary valve is replaced with dual control valves 14 and 15 on theoutput and input conduits respectively, with motor driven control valve14 serving as the frequency modulation valve.

Now by the use of this invention a cryocooler, i.e. a pulse tube system,may operate at its most efficient frequency rather than being limited tooperating at the frequency of the compressor while also avoidingcomplications caused by oil contamination of the pulsing gas. Althoughthe invention has been described in detail with reference to certainpreferred embodiments, those skilled in the art will recognize thatthere are other embodiments within the spirit and the scope of theclaims.

1. A method for operating a low frequency cryocooler system comprising: (A) generating pulsing gas at a frequency of at least 25 hertz by compressing a gas using a moving element moving proximate a surrounding wall wherein no oil is employed between the moving element and the surrounding wall; (B) passing the pulsing gas through a frequency modulation valve and reducing the frequency of the pulsing gas to produce lower frequency pulsing gas; and (C) passing the lower frequency pulsing gas to a regenerator which is in flow communication with a thermal buffer tube.
 2. The method of claim 1 wherein the moving element is a piston driven by an axially reciprocating electromagnetic transducer.
 3. The method of claim 1 wherein the pulsing gas is passed through a discharge frequency modulating volume prior to being passed through the valve.
 4. The method of claim 3 wherein the discharge frequency modulating volume includes a reservoir.
 5. The method of claim 1 wherein the lower frequency pulsing gas has a frequency of less than 10 hertz.
 6. A low frequency cryocooler system comprising: (A) a compressor having a discharge and having a moving element proximate a surrounding wall wherein no oil is employed between the moving element and the surrounding wall; (B) a regenerator, a frequency modulation valve, discharge conduit extending from the discharge to the frequency modulation valve, and regenerator input/output conduit extending from the frequency modulation valve to the regenerator; and (C) a thermal buffer tube in flow communication with the regenerator.
 7. The low frequency pulse tube system of claim 6 wherein the compressor is a linear compressor and the moving element is a piston driven by an axially reciprocating electromagnetic transducer.
 8. The low frequency pulse tube system of claim 6 wherein the frequency modulation valve is a rotary valve.
 9. The low frequency pulse tube system of claim 8 further comprising suction conduit extending from the rotary valve to the compressor suction.
 10. The low frequency pulse tube system of claim 6 further comprising a reservoir positioned on the discharge conduit between the discharge and the frequency modulation valve to comprise a discharge frequency modulating volume.
 11. The low frequency pulse tube system of claim 9 further comprising a reservoir positioned on the suction conduit between the rotary valve and the compressor suction to comprise a suction frequency modulating volume. 